Hi, have a requirement for the roller and the ball bearing set on the WM20 timing side of the cases. Could you please look at the attached photos and confirm to me that these are indeed the roller bearing part # 24-722 gear side and the ball bearing part # 27-261 as original replacements?
Requesting this info as I am not sure of the military part numbers listed. Thanks for your time. On the Roller, the info reads:
16D.....
LV6/MT7-6646
Bearing Roller Journal
Single Lipped Outer
7/8 x 2 1/4 x 11/16
email (option): unpob@yahoo.com
Hi Henri..That is the timing side roller, WD Number MT7/6646...Timing side ball WD Number is MT7/6017...Ian
email (option): ian@wright52.plus.com
hi douglas,yes i think the same way as you,in later years especially when bsa went down the route of relacing bearings with bushes as in the unit construction examples.im not familiar with the pre war set up as regards crankshaft bearings but i think the ministry of supply may have had a hand in this.i have no expertise with the bike details ,however all machine tools were uprated and manufactured to exacting standards and inspected by ministry people, a plate then affixed stating ,to the wording of,this machine is built to the requirements of ''war finish and is painted accordingly''.exception churchill machine tools manchester(probably the finest machines ever built on this planet)their plate states,this machine is painted in accordance with war finish,workmanship and accuracy are standard.cheers rick 
email (option): richardholt@rocketmail.com
Both the sidevalve and overhead valve models had three bearings when introduced pre war..The key part of the 1938 redesign of the sidevalve was to introduce an air gap behind the tappet chest to prevent overheating of that side of the barrel and consequent distortion of the bore.
That meant a wider barrel casting, with the valves positioned further from the cylinder (an unfortunate side effect of the change to the barrel from the point of view of gas flow and combustion). A wider cylinder head and an increase in the number of head bolts from 8 to 10 matched this layout.
It was then necessary to move the timing chest out by the same amount to realign the tappets...so the crankcase was made wider on the timing side. The improvements to the timing gear assembly to ease manufacture, assembly and maintenance were also carried out as part of this redesign of the crankcase. The drive side remained unaltered.
The wider crankcase dictated a lengthening of the timing side mainshaft..and to support the longer shaft another bearing was added on that side.
So the extra bearing was not about additional strength or anything to do with power output but was a consequence of the changes made to achieve improved cooling!....Ian
email (option): ian@wright52.plus.com
Hi Douglas..Cylinder head volumes and hence compression ratios are pretty much identical on both types, though the shapes differ. It would be interesting to see if the narrower chamber of the early type would tolerate more compression and breath a little better than the later one..it is quite possible it would..Ian
email (option): ian@wright52.plus.com